Source : Webster's Revised Unabridged Dictionary (1913)

Planetoid \Plan"et*oid\, n. [Planet + -oid.] (Astron.)
   A body resembling a planet; an asteroid.

Source : WordNet®

planetoid
     n : any of numerous small celestial bodies composed of rock and
         metal that move around the sun (mainly between the orbits
         of Mars and Jupiter) [syn: {asteroid}, {minor planet}]